Following the stacktrace, the error can be reproduced in the line
zeros(eltype(sol.u), length(sol.u))
and the error appears to be coming from the fact that Julia doesn’t know what to do about making a vector of zeros with a Vector element type (maybe because of issues of not having information of size to allocate?). My initial thought would be to use StaticArrays, but in this post @ChrisRackauckas indicates that they aren’t appropriate for ODE solving. Maybe somebody has ideas if there is a way to package the initial guess sol.u, maybe as a Matrix instead of as Vector{Vector{Float64}} or something similar?